1. Field of the Invention
This invention relates to an electronic instrument having a non-volatile display, and more particularly to an electronic instrument in which appropriate ON-OFF switching of the power source for driving the non-volatile display and appropriate clearing of the display can be effected.
2. Description of the Prior Art
Non-volatile (or persistent) display means, or display means having a memory capability once display information has been written therein, can maintain contrast without volatilizing (or disappearing) even if the power supply thereto is cut off. This leads to various advantages such as reduced power consumption, positive information output, reusable soft copy, etc., and applications of such display means have therefore been studied extensively in recent years. Non-volatile display means are particularly well suited for use in computer output devices, camera viewfinders, viewers for microfilm readers (reader-printers), soft facsimile paper, etc., or editting drums in copying machines or the like.
However, incorporation of non-volatile display means into electronic instruments poses various problems. For example, a solution must be reached as to what specific measure is to be used to effect ON-OFF action of the driving power source for the volatile display means, and what timing and what specific measure is to be adopted to clear the display means. Further, consideration must be given to the correlations of semiconductor control circuitry for effecting the drive control of the non-volatile display means with the clearing action, viz. optimum selection and combination of the power source ON-OFF switching of the clear signal generating system, the control circuit and the non-volatile display means. Moreover, these considerations must be taken in conjunction with an optimum design whereby the electronic instrument is manually operated in actual use, so that greater ease of manipulation may be provided. For example, after an electronic instrument with non-volatile display means has been used, and when the power switch thereof has been opened, the image information previously displayed by the display means can be kept unchanged. But when the electronic instrument is again used, the previous image information is no longer necessary and a clearing action must, then be effected on the display means.
Also, even during use of the instrument, if the content of the display image has to be changed, no new image information can be written in until a clear signal is specially applied to the display means. Particularly, in portable electronic calculators or the like, operated numbers, operating numbers, results, etc. are all displayed by a single, plural-digit numerical display device. Thus, if it is desired to employ non-volatile display means therefor, successive clearing actions would be required and the provision of a special clear key for clearing the non-volatile display means would make the manipulation cumbersome. In the case of battery-driven portable calculators, battery power saving must be taken into account, and this could be expedited by the use of non-volatile display means. However, in order to satisfy such requirements overall considerations must be given to the battery ON-OFF timing and its operation with the non-volatile display means, the clear timing and its connection with the non-volatile display means, and even the clear timing and its connection with the semiconductor control circuit governing the drive control of said display means and arithmetic operations, as well as the ON-OFF timing of the battery as the power source for all of these circuits.